Computer and data processing systems deal with many different types of information. Often, this information is retrieved from one place and used in another. Therefore, communication procedures and facilities frequently drive important system design decisions.
In a general-purpose system, the communication facilities may be called upon to carry data with widely varying delivery constraints. Those constraints can be analyzed broadly in terms of integrity and timeliness requirements. For example, some data is expected to be transferred without error, but can tolerate transmission delays to ensure correctness. Other data can tolerate some errors, but must be delivered before a deadline. Information that must be delivered both quickly and correctly may place the most stringent demands on a communication subsystem.
Communication subsystems themselves are often designed to move data as quickly as possible. Even if the communication fabric cannot guarantee data integrity, higher-level protocols can be provided to detect and correct errors. Faster data transmission capability is usually considered to be an unalloyed benefit. However, for some types of data, raw transmission speed is less important than regularity and predictability. For example, multimedia data—that is, data to reproduce audio and/or video signals—may be more sensitive to late delivery than to data errors. If multimedia data arrives late, playback may stall. In contrast, data errors may result in visible or audible glitches, but these may be less intrusive than jerky, stop-and-start performance seen with delayed data.
The characteristics of multimedia data illuminate another communication facility requirement as well. Since a multimedia program is usually replayed in a standard time order, and often in realtime, early delivery of data is not useful. In fact, early-delivered data may increase a system's resource usage, since the data must be stored until it is time to play it.
Multimedia data provides a good example of an isochronous data stream: data to be played back should be received regularly, and neither early nor late. Methods of operating a general-purpose communication facility to achieve these qualities in a data stream may be useful in improving the performance of systems that rely on isochronous data.